1. Field of the Invention
The present invention relates generally to failure analysis and device qualification in integrated circuits, as well as design debugging. More particularly, the present invention relates to an improved method of locating specific integrated circuit current paths that are related to defects, mis-processing or poor circuit design.
2. Description of Related Art
In integrated circuit (xe2x80x9cICxe2x80x9d) manufacturing, parts are tested throughout various stages of fabrication and packaging. Some parts may not pass the required tests or may otherwise be deemed defective. For example, if an input/output pin on the IC has a current leakage path it may be deemed defective. Many times in developing and manufacturing ICs it is difficult to determine exactly what causes the defective part to fail the required tests. IC technology trends only exacerbate this problem as more routing layers are required to connect tightly packed circuit elements. In addition, the number of transistors in each successive generation of ICs are doubling approximately every eighteen months, which results in a continual increase in total current demand with each new generation of ICs. Methods for determining the root cause of the defective ICs are a vital part of developing and manufacturing integrated circuits.
Fortunately, IC analysis apparatuses and methods exist which provide useful insight into design, manufacturing, and processing defects, as well as other failure modes. IC analysis includes the location, identification, and mapping of an IC to reveal defects. For example, different methods for IC analysis are described in, xe2x80x9cNovel Failure Analysis Techniques Using Photon Probing With a Scanning Optical Microscope,xe2x80x9d by E. I. Cole Jr. et al. Of particular interest is the Light-Induced Voltage Alteration (xe2x80x9cLIVAxe2x80x9d) method.
In addition, U.S. Pat. Nos. 5,430,305 and 6,078,183 disclose LIVA and Thermally-Induced Voltage Alteration (xe2x80x9cTIVAxe2x80x9d) techniques for ICs respectively. These techniques power the entire IC with a constant current source (the voltage supplied to the IC may vary) while scanning the surface of the IC with a light source of specific wavelengths that depend on whether LIVA or TIVA is used. Localized current changes are induced due to photon exposure (LIVA), or localized heating of the IC (TIVA). These local current changes alter the total power required, and these changes are more pronounced when the light source is exposed to a defect on the IC. Since the IC is powered by a power supply with constant current and variable voltage, any alteration in the total power requirement causes a voltage alteration on the power supply. Therefore a voltage alteration on the power supply caused by light exposure may indicate that a defect in the area of the IC that is being exposed to the light source.
However, these methods have weaknesses that cause defects to be overlooked. For example, in a typical setup, the IC is tested with various capacitors connected to the power supply to filter out noise injection from the power supply. Besides filtering noise, the capacitors attenuate the induced voltage alteration, thereby allowing defects to remain unnoticed. Another problem is that circuit elements that are only marginally defective may not produce enough of a current change when exposed to the light source to overcome the noise floor and be noticed. With millions of circuit elements (specifically transistors) in a typical IC, the overall noise floor can be quite high. This remains true despite the fact that each transistor may be in a quiescent state and will draw only a minute amount of current individually. In such circumstances, a marginally defective transistor that produces less of a current change than a fully defective transistor with respect to the overall quiescent current draw is more likely to remain unnoticed. In addition, because current demands (both quiescent and dynamic) in successive generations of ICs increase every eighteen months, the current risk becoming less and less capable of detecting defects. Accordingly, a solution to these problems is needed.
The present invention provides an improved defect detection technique and system. In a preferred embodiment, the defect detection system includes two separate power supplies: a global power supply and a dedicated power supply. The global power supply provides power to the IC and is preferably a constant voltage power supply. The dedicated power supply supplies power to specific points of interest on the IC, and may be a variable current and/or a variable voltage power supply. The dedicated power supply may be configured to supply a constant current approximately equal to a previously measured value while letting the voltage vary. Next the IC is scanned with the light source, and the defect causes a change in power demand on the dedicated power supply when exposed to the light source. This change in power demand is the result of photon exposure or localized heating of the IC, and is more of a drastic change when the light is exposed to a defect on the IC. The change in power demand causes a voltage alteration on the dedicated power supply, thereby allowing a defect to be spotted.
The present invention is expected to alleviate the problems described above because the global power supply provides the quiescent current to the IC. The separate dedicated power supply then has a lower noise floor and is more sensitive to voltage alterations, especially from marginally defective devices. Therefore, a voltage alteration on the dedicated power supply is measured independent of any background noise resulting from the quiescent state of the IC. Also, if the dedicated power supply supplies power only the suspect area of the IC, then a greater sensitivity is achieved when the laser is scanned over that area. This same premise is expected to provide promising results even as new generations of ICs are developed which require more current.